Circuits with analog functions, as opposed to purely logic circuits, often require the presence of biasing circuits that define sources of current with a well-defined value.
Typically, these biasing circuits use current mirror cells with feedback loops: a branch of the mirror imposes a current of forced value onto another branch that itself imposes a current of forced value onto the first branch. It is this type of feedback loop that allows a relatively stable operating condition to be defined, notably a current whose value is well defined with respect to temperature. Conventional biasing circuits are of the ‘bandgap’ type or of the PTAT (Proportional To Absolute Temperature) type.
Unfortunately, these biasing circuits have a drawback which is the random nature of their starting configuration at power up or after an abnormal interruption in operation (power supply power glitch or other interference). This random nature is explained by the fact that, aside from their stable operating point for which they exhibit the desired characteristics (in particular, as a function of temperature), they possess another undesirable stable operating point with zero, or virtually zero, current (in other words, an operating point different from that at which they must remain in normal operation). In the absence of power, or when power is re-applied, there is a risk of them staying at this undesirable operating point and not being able to spontaneously move away from it.
For this reason, it is desired to add an auxiliary starter circuit onto these circuits which forces a current to start flowing in the biasing circuit at the moment that power is re-applied, and which therefore forces this circuit toward its normal operating point by driving it away from the undesirable operating point.
Starter circuits used in this context are of two types:                those which must be activated by a specific start-up command; these can have a low standby current consumption and a high current consumption during the start-up period which is very short; these circuits therefore have the advantage of drawing very little current in the steady state, but they have the drawback of not being able to start automatically when the power is re-applied;        those which can start automatically in the presence of a power supply, whether that be at initial power-up or when the power is re-applied after a power glitch, or following any other interference event which might have interrupted the normal operation of the biasing circuit; these circuits generally have the drawback of drawing a non-negligible steady-state current.        
The problem is that the current consumption is a parameter that is becoming increasingly important in many applications and especially in all applications that operate on small batteries (mobile telephones, etc.).
The aim of the invention is to provide a circuit for automatic starting which can both restart automatically at power up or following a power supply interruption and which draws very little current in the steady state.